Copyright is a form of intellectual property law that gives the creator of an intellectual work the right to control how that work may be used. In its usual form, that control comes in the way of restrictions: copyright works to restrict others from using the intellectual works of others.

People who are anti-copyright believe that the current copyright system and/or any copyright system is inappropriate and should be severely curtailed and/or abolished altogether. If you are anti-copyright, you probably will want to release your work into the public domain. See the next section.

Copyleft is the idea of using the copyright system to remove the common restrictions used in copyright. People who advocate for copyleft may or may not be anti-copyright.

If you believe in copyleft, you will probably want to use one of the free licenses detailed in the final section.

The open source model for software programs is well-known, and has been successful in a range of applications, from entire operating systems (Linux) to the software that is used to run Wikipedia and its sister projects. The key features of open source are

ability to view and modify the source code

requirement that any modified versions of the software are released under the same license.

Richard Stallman is an American software developer who founded the Free Software Foundation and wrote the GPL (GNU General Public License), the most widely-used open source license for software. The Free Software Foundation designed the GNU Free Documentation License, originally intended to do the same for software documentation (help files etc.) as the GPL does for software itself.

The Free Cultural Works Definition defines four essential requirements for a license to be considered a "Free Culture License":

The freedom to use and perform the work

The freedom to study the work and apply the information

The freedom to redistribute copies

The freedom to distribute derivative works

The Wikimedia Foundation (which manages this project, Wikimedia Commons, as well as most famously Wikipedia) is currently one of the largest proponents of (and thus indirectly advocates for) copyleft licenses. Their licensing policy requires that all projects they manage use "Free Culture Licenses" as defined by the Free Cultural Works definition.

Flickr, a popular photo-sharing website, allows users to easily choose Creative Commons licenses for their photographs. Flickr's popularity has increased awareness and knowledge of the Creative Commons licenses among its users. Flickr's API also helps to encourage re-use of such freely-licensed photographs.

When you release a work into the public domain (or otherwise attempt to renounce all rights to the work), you are giving up all control over the work. In some jurisdictions it may not be legally possible since moral rights may be unrenounceables, but one can waive rights of exploitation and distribution.

This means that although a public domain work is in one sense "the most free", as it can be used in the widest range of possible uses, derivative works may "become unfree" as the creator of the derivative work can choose to use traditional restrictive licensing.

To ensure that works "stay free", it's necessary to use a work that has a ShareAlike condition.

There are two situations where media can be re-used. The first one is for more or less "personal use". For these users, a license with a FullText requirement is somewhat unfriendly. Another one is a user contributing to a collaborative site such as a wiki. In this case, the entire site likely already has a chosen license and all content contributed to the site must conform to this license. For these users, because no licenses are compatible, they need material to be available under the same license as the original.

This is why Commons recommends dual-licensing under the GFDL and the CC-BY-SA license (all versions). Both licenses have a ShareAlike restriction, ensuring the work will remain free no matter how it is used or modified. Using 'all versions' of the CC-BY-SA license maximises re-usability for sites which may be "stuck" with an earlier version of the CC-BY-SA license. Using the GFDL ensures the work can be used unquestionably by the vast majority of Wikimedia projects.